中文摘要：

【目的】研究黄土丘陵区典型农作物玉米和谷子生育期内土壤临界剪切力的季节变化特征及其影响因素,以期为该区的粮食安全生产和土壤侵蚀过程模型建立提供参考依据。【方法】以黄土丘陵区玉米地和谷子地为研究对象,在6组不同的水流剪切力条件下（τ=5.71—17.18 Pa）,借助坡面径流冲刷试验测定土壤的分离能力,结合土壤侵蚀过程WEPP模型,利用线性回归方法推求土壤临界剪切力（τc）,分析了玉米和谷子生育期内土壤临界剪切力的季节变化规律。【结果】黄土丘陵区玉米和谷子生育期内土壤临界剪切力具有明显的季节变化规律（P〈0.05）,整体呈上升趋势。玉米地τc表现为先降低后升高再降低又升高的季节变化模式,变化范围为1.51—4.89Pa,平均值为3.0 Pa,最小值（1.51 Pa）出现在五叶期,最大值（4.89 Pa）出现在收获期;谷子地τc表现为先降低后升高的季节变化模式,变化范围为1.06—6.53 Pa,平均值为2.93 Pa,最小值（1.06 Pa）出现在幼苗期,最大值（6.53 Pa）出现在成熟期。玉米地和谷子地土壤临界剪切力的季节变化由土壤黏结力、初始含水量等土壤属性的季节变化和作物根系生长所致。两种作物地的土壤临界剪切力与土壤黏结力、初始含水量和作物根重密度间呈正相关关系。在玉米和谷子生育期内,利用土壤黏结力、初始含水量和作物根重密度可以较好地模拟两种作物地土壤临界剪切力的季节变化（R^2〉0.74,NSE〉0.72）。【结论】作物根系、土壤黏结力和初始含水量等的季节变化是影响玉米地和谷子地土壤临界剪切力季节变化的主要因素。土壤临界剪切力与根重密度、土壤黏结力和初始含水量间呈正相关关系。用根重密度、土壤黏结力和初始含水量等参数能够较好地模拟玉米地和谷子地土壤临界剪切力的季节变化。

英文摘要：

【Objective】The aim of this study is to explore the seasonal variations of soil critical shear stress under two typical farmlands in the hilly region of Loess Plateau in order to provide a reference for ensuring food security and establishing a soil erosion process model.【Method】Maize and millet lands in the hilly region of Loess Plateau were used as the study plots. The scouring experiments were carried out in a laboratory flume to simulate soil detachment by concentrated flow under six different shear stresses ranging from 5.71 to 17.18 Pa. The soil critical shear stress（τc） was estimated by the soil erosion process WEPP model. The seasonal variation of soil critical shear stress were discussed under maize and millet farmlands during the crop growing season.【Result】Results show that soil critical shear stress under typical farmlands in the hilly region of Loess Plateau displayed significantly seasonal variations（P〈0.05）, which increased during the whole growing season. During the growing season, soil critical shear stress in maize land displayed a pattern of the seasonal variation following declined firstly, then increased, declined again, and rose. τc values varied from 1.51 Pa to 4.89 Pa, with a mean value of 3.0 Pa. The minimum and maximum values of soil critical shear stress for maize appeared at the five-leaf stage and at the harvesting stage, respectively. Soil critical shear stress in millet land displayed a pattern of seasonal variation following declined firstly, then increased. τc values varied from 1.06 Pa to 6.53 Pa, with a mean value of 2.93 Pa. The minimum and maximum values of soil critical shear stress in millet land appeared at the seedling stage and at the maturing stage, respectively. The seasonal variations of the soil critical shear stress of maize land and millet land were influenced by the seasonal change of soil cohesion and initial water content, and crop root growth. There were positive relationships between soil critical shear stress and soil cohesion, initial